This invention relates to hyperspectral imaging and, more particularly, to correcting color images for keystoning and smile.
Present day imaging optical systems form images by attempting to focus the wavelengths of light passing through the lens system into the smallest possible volume. Refractive imagers may be made with differing degrees of optical correction for both monochromatic (spherical, astigmatism, coma,) and polychromatic aberrations. Lens radial distortion, often called pin-cushion or barrel distortion, is another source of image distortion and induced vertical parallax. Lens radial distortion is caused by the use of spherical lens elements, resulting in the lens having different focal lengths at various radial distances from the center of the lens. Increasing focal length from the center of the lens is called pin-cushion distortion and the reverse is called barrel distortion. Parallax among the object plane, the lens plane and the image (film or CCD) plane affects image perspective and produces the keystone distortion effect sometimes experienced.
Conventional spectrometer lens design practice operates on the assumption that a slit is the object, a dispersive element is sandwiched between a collimating lens and an imaging lens, and an image is formed on a detector oriented perpendicular to the optical axis. The resultant image that is formed consists of multiple images of the slit that are spread out in the color direction. Each slit image corresponds to a different wavelength. Two aberrations or distortions are commonly found in these designs. One distortion may be termed xe2x80x9cchromatic keystonexe2x80x9dxe2x80x94an effect that causes a line of each color to differ in length depending on where the ray propagates with respect to the center of the lens. The second distortion caused by these designs is curvature of the slit, so that light of a single color does not form a straight line, but is instead curved up (xe2x80x9csmilexe2x80x9d), or down (xe2x80x9cfrownxe2x80x9d). This distortion arises at the dispersing element when the ray bundles exit the dispersing element at compound angles relative to the flat surface. Often, the amount of curvature varies with wavelength. It would be of great advantage to provide an optical system in which both of these distortions are removed.
In accordance with the principles of the invention, an image acquisition system scans an object and passes it through an imaging spectrometer whose imaging optics are deliberately designed, advantageously using commercially available optical design software, to be chromatically dispersive rather than attempting to be apochromatic. The lens system of the imaging spectrometer accepts light from the object that has passed through a slit, collimates it and, instead of focusing all of the wavelengths in the transmitted light into the smallest possible size in a single focal plane, separately focuses each wavelength into a respective focal plane. The position of the focal plane for each wavelength is proportional (either linearly or non-linearly) to wavelength. For example, the middle wavelength may be focused at one position, the shortest wavelength focused at a second position, and the longest wavelength focused on the other side of the central wavelength. A new, color-corrected focal plane is defined by the collection of the focal planes respective to each color and a suitable device such as a CCD may then be positioned at the color-corrected focal plane to acquire an image that is free of color keystoning, smile or frown.